In a light-water reactor nuclear power plant, austenitic stainless steel, such as SUS 316L, SUS 304 or SUS 347, is used as a structural material for equipment, piping or the like, including a reactor pressure vessel and reactor internal components. In an operation of joining the structural materials by welding, intergranular corrosion or stress corrosion cracking is likely to occur in a heat-affected zone adjacent to a weld. The stress corrosion cracking is known as a phenomenon which occurs when an environmental condition, a stress condition and sensitization in the material are superimposed.
In this specification, the term “sensitization of austenitic stainless steel” means a phenomenon that, when the stainless steel is heated at a temperature of 450 to 800° C. for a long period of time, solid-solved carbons are precipitated as chromium carbides in grain boundaries, and thereby a region deficient in solid-solved chromium is formed in the vicinity of the grain boundaries, which causes deterioration in corrosion resistance of a steel member. After welding and joining austenitic stainless steel members together, a welding heat-affected zone between a weld toe and a base metal is sensitized due to a welding heat input.
With a view to enhancing plant reliability so as to improve a plant operating rate, various measures against the stress corrosion cracking have heretofore been studied.
For example, as measures from the viewpoint of material, there have been known a technique of reducing a carbon content in a steel member to make precipitation of chromium carbides less likely to occur, and a technique of subjecting welded members to a solution heat treatment in their entirety to improve a sensitized microstructure caused by welding and relax a welding residual stress (see, for example, the following Patent Document 1). However, these sensitization measures involve a problem that it is necessary to adjust a chemical composition of a steel member as a base metal or to perform a heat treatment after welding in a production plant, which is highly likely to lead to an increase in cost.
Further, as one example of measures from the viewpoint of welding procedure, there has been known a technique of, in an operation of joining a plurality of stainless steel or nickel-based alloy pipes by welding, welding the vicinity of each of a pair of toes of a final weld layer at a lower heat input level (see, for example, the following Patent Document 2). However, this technique is designed to modify only a welding condition in a final stage of the welding. Thus, it is undeniable that only a limited effect is expected.
As another example of the measures from the viewpoint of welding procedure, there has been known a pipe-inner-surface overlay welding (build-up welding) technique of coating a sensitized region of an inner surface of a pipe contactable with liquid, with deposited metal excellent in stress corrosion cracking resistance (see, for example, the following Patent Documents 3 to 7). However, welding heat from the overlay welding gives rise to formation of a new heat-affected zone in a base metal. For example, if the pipe is subjected to a prolonged heat treatment after the overlay welding in order to eliminate the heat-affected zone, a problem will occur that a compressive stress generated by the overlay welding declines.
As yet another example of the measures from the viewpoint of welding procedure, there have been known a water cooling technique of cooling an inner surface of a pipe by water flowing therethrough during welding, to relieve sensitization in the inner surface of the pipe and shift a welding residual stress toward a compressive side, and a technique of joining a plurality of pipe members together by welding and then subjecting an outer surface of the obtained pipe to high-frequency induction heating while cooling an inner surface of the pipe, so as to shift a residual stress in the inner surface of the pipe toward a compressive side to prevent the stress corrosion cracking (see, for example, the following Patent Document 8). However, in these techniques, the outer surface of the pipe has a residual stress in a tensile direction. Thus, in cases where the outer surface is also in contact with cooling water as in nuclear reactor internal piping, there is a problem that it is impossible to prevent the stress corrosion cracking in the outer surface.
Further, as measures to be applied to equipment, piping or the like in a currently operating nuclear plant, there has been known a repair welding technique, wherein, after removing a defective portion, a periphery of a single strapped joint attached onto a surface of the removed portion is welded and joined (claded) to a base metal (see, for example, the following Patent Document 9). However, a conventional cladding material does not have sufficient stress corrosion cracking resistance, and thereby a cladding region is likely to be sensitized.
In addition, as measures to be applied to a previously-welded pipe being used, there has been known a technique of simultaneously preventing the stress corrosion cracking in inner and outer surfaces of a pipe (the following Patent Document 10). More specifically, it is a method designed for preventive maintenance using a laser irradiation unit which is adapted to perform a rapid solidification process for forming a rapidly-solidified microstructure in an inner surface of a target member, while reducing a residual stress in an outer surface of the target member, or a solution heat treatment for reducing respective residual stresses in an inner and outer surfaces of a target member. However, due to changes in a laser irradiation power and a moving speed of a laser beam to be output from the laser irradiation unit, this technique is likely to fail to sufficiently shift a residual stress in the outer surface of a weld toward a compressive side. Moreover, if the laser irradiation power becomes excessively large, the laser irradiation is likely to cause a new sensitized region. In cases where this technique is applied to nuclear reactor internal piping, when the piping undergoes high-dose neutron irradiation during nuclear reactor operation, helium (He) produced by a nuclear reaction will be accumulated inside the piping. Thus, there is concern that He-induced cracking occurs when a large amount of heat is applied to a weld of the piping.
The Patent Document 6 describes a stress-corrosion-cracking prevention technique, wherein, in an operation of butt-welding two stainless steel pipes, a melting/solidification process or an overlay welding process is performed in a depth range of 0.1 mm to 1.0 mm from an inner or outer surface of a base metal, to form a solidified layer. However, the solidified layer has the same chemical composition as that of the base metal. Thus, even after implementation of this technique, the welded members are still in a situation where the stress corrosion cracking resistance is intrinsically poor.
As above, the conventional stress-corrosion-cracking prevention techniques and the conventional repair techniques are just intended to: perform welding using a commercially-available welding material so as to prevent damage of a weld joint itself; coat a final layer of a weld joint with a commercially-available stress corrosion cracking-resistant alloy; clad a heat-affected zone generated in a weld joint itself, with a commercially-available cladding alloy; or re-weld a cladding region, or a portion of a weld joint including a non-welding base metal subjected to removal of cracking or other defect, using a commercially-available welding material. Newly deposited welding material itself does not have a sufficient stress corrosion cracking resistance, during a long-term plant operation, particularly, in a welded junction or the vicinity thereof, in a region having a high electrochemical potential or a region irradiated with a high neutron flux, more particularly, in a bead along a weld edge where the commercially-available welding material is diluted with a composition of the base metal. Thus, the conventional stress-corrosion-cracking prevention techniques and the conventional repair techniques have a problem of re-occurrence of the stress corrosion cracking.
Further, the use of an existing an overlay-welding material involves the following problems. Characteristics necessary for a weld joint include a stress corrosion cracking resistance of a heat-affected zone, and a cracking resistance of a deposited metal zone. It is also necessary for an overlay weld to have a cracking resistance. However, in cases where overlay welding is performed to form a multi-layer overlay weld, i.e., two or more weld layers, bending crack often occurs in a first weld layer or an underlaying weld layer. Therefore, in existing welding materials widely used for overlay welding, such as Y308 series, a composition thereof is adjusted to form δ-ferrite or the like so as to become a deposited metal improved in solidification cracking sensitivity. However, in overlay welding using the existing overlay-welding material having a composition adjusted in the above manner, a problem will occur that a corrosion resistance of a welded junction is deteriorated due to formation of δ-ferrite phase, although the cracking resistance of the welded junction is improved.